The present invention relates generally to an impedance matching means, and more particularly to an impedance matching means between an antenna and a transmission cable.
The continuing growth in wireless communications has spurred the demand for more radio frequency (RF) antennas for use in notebooks, portable handsets, and other products. When employing an RF antenna, it is important to match the impedance of the RF antenna load to the impedance of the antenna feed cable, especially when the antenna is fed by an unbalanced transmission cable, such as a coaxial cable. Coaxial cables are normally designed to provide a 50 ohm or a 75 ohm normative resistance to the antenna. However, the input impedance of the antenna will be changed by the antenna attachment mechanism. It is often found that the input impedance of the antenna is non-matching when measured from the coaxial cable feed point.
The input impedance of an antenna can be expressed by Zi=RA+jXA, wherein RA and XA are respectively the real and imaginary parts of the input impedance. In order to attain favorable antenna radiating efficiency, the loss produced by the imaginary part XA must be eliminated. It is conventional to use a balun, such as a quarter wavelength balun, to perform impendence matching of an antenna. The balun prevents asymmetrical loading of the antenna and a concurrent induction of a current on the exterior of the unbalanced transmission cable. Impedance matching is achieved by designing the real part RA of the antenna load to be substantially equivalent to the characteristic impedance of the transmission cable, while at the same time, selecting the impedance of the balun to offset the imaginary part XA of the antenna load. However, complex calculations are needed to predetermine where the balun should be disposed in the antenna.
Hence, an improved impedance matching means between an antenna and a transmission cable is needed.
A primary object of the present invention is to provide a convenient impedance matching means between an antenna and a transmission cable.
An impedance matching means according to the present invention is used with a printed wire board (PWB) antenna. The PWB antenna includes a PWB comprising an upper side and a lower side, with an antenna body printed on the upper side. The antenna body comprises a radiating portion, a feed portion, and a ground portion. A transmission cable is connected to the antenna body with its inner conductor soldered to the feed portion and its outer shield soldered to the ground portion. The impedance matching means is electrically connected to the feed portion and to the ground portion and includes a first conductive patch and a second conductive patch. The first and the second conductive patches are disposed parallel to one another on the upper side and on the lower side of the PWB, with the PWB sandwiched therebetween. The impedance of the antenna can be conveniently predetermined to match a preselected cable by either changing the relative areas of the two patches or by changing the relative permitivity of the dielectric material of the PWB.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.